Damage to articular cartilage is an exceedingly common problem affecting the joints of millions of people. The ability of adult cartilage to regenerate is limited due to avascularity and the absence of stem cell in this tissue. Although defects that extend to subchondral bone provoke the formation of a fibrous or fibrocartilagious tissue, it undergoes premature degeneration because the repair tissue is biochemically and biomechanically different from hyaline cartilage.
A variety of clinical procedures have been developed to repair articular cartilage defects, but success has been limited. Bone marrow stimulation technique (fine joint plastic surgery and drill articular plastic surgery) interpenetrate subchondral bone, and stimulate multipotent stem cells within bone marrow to repair the defects into a fibrous tissue or fibrocartilage. However, these methods have disadvantages that the repair fibrocartilage lacks such biochemical and biomechanical property as normal hyaline cartilage has. Further, their effects have not been proved yet for the large area defects, osteoarthritis and the older patient. Osteochondral/chondral graft is simple and effective for small size defect, but if the tissue is moved from low weight part to high weight part, it may cause a harmful compression due to non-physiological bearing in the transplanted position. Periosteal and perichondrial graft has a potential advantage to introduce new cell assembly which is able to construct cartilage, but has also disadvantage that hyaline-like repair tissue tends to undergo calcification through osteogenesis in cartilage.
Furthermore, several cell-based procedures using chondrocytes, mesenchymal stem cells (MSCs), periosteocytes and perichondriocytes have been developed. Chondrogenic progenitor cells such as MSCs, periosteocytes and perichondriocytes get popularity more and more as potential cell sources to repair the osteochondral defects. However, the progenitor cells capacity to construct articular chondrocytes is limited, and the age of patient is directly related to clinical results. Further, it is reported that the repair hyaline-like tissue tends to undergo calcification.
Autologous articular chondrocyte transplantation (ACT) has been clinically applied in small defects of articular cartilage, but only limited success has been reported. Even though articular chondrocytes can be easily isolated from mature articular cartilage by enzyme digestion, it requires two step-procedures for harvesting and grafting which are invasive to joint and considerably expensive. Thus ACT cannot be applied to more than two small lesions, to the lesion size larger than 10 cm2, to the patient of rheumatoid or immune-related arthritis, and to the older patient due to articular cartilage degeneration with age.
Furthermore, only one to two percentage by the volume of articular cartilage is chondrocyte, in which approximately 2,000 cells/mg of human articular cartilage can be isolated for culture. An average 3 cm2 defect requires 9×106 cells for the ACT procedure if implanted at cell densities similar to that found in the normal human knee joint. In reality, more cells may be required for ACT since 26% of patients under the age of 40 with grade IV chondromalacia lesions had multiple lesions. Therefore, a large number of chondrocytes are required to fill adequately a volume of defect with a similar cell density as seen in normal human articular cartilage.
During serial monolayer culture for cell expansion, chondrocytes tend to stop expressing cartilage-specific proteoglycan and type II collagen, and switch to express type I collagen with producing a small amount of proteoglycan. Such dedifferentiation is a main problem which limits cell expansion in vitro and ACT application.
Further, costal cartilage is the biggest permanent hyaline cartilage in the mammalian body, which has been suggested as a possible alternative donor source for autologous graft in reconstruction of articular cartilage, external ear and trachea. Costal cartilage has been used to repair osteochondral defects in small joint such as the interphalangeal joints and the temporomandibular joints. Costal cartilage seems to have several advantages over articular cartilage as donor tissue. Actively proliferating chondrocytes were detected even in patients 80 years of age or older, and a significant amount of costal cartilage is available in patients younger than 60 years. In addition, costal cartilage is abundant, and its easy surgical accessibility allows less harm to donor site. Therefore, if costal cartilage has the same phenotype as hyaline cartilage of articular cartilage, it can be considered the most useful source for treating a variety of articular cartilage disorders such as rheumatoid arthritis, osteoarthritis and cartilage defects of the joints.
However, so far, only autologous tissue transplantation to graft costal cartilage per se to articular cartilage part has been performed, and there has been no effort to isolate chondrocytes from costal cartilage to regenerate articular cartilage by tissue engineering method.
Furthermore, transplantation of chondrocytes or chondrogenic cells alone has shown to be successful in rabbit models, but the healing rate has limited due to loss of their viability in the transplanted cells and due to the difficulty of fixing chondrocytes in the defect. To overcome the difficulties related to the surgical procedure and to find a method of maintaining chondrocytes in the defect without outflow of the cells in the articular cavity, new approaches in different biomaterial carriers as scaffolds onto which cells are seeded, have been studied. Ideal scaffolds should be biocompatible, bioabsorbable or remodeled, and provide framework that facilitates new tissue growth. They should also display material and mechanical properties compatible with articular cartilage function. A variety of biomaterials, naturally occurring such as collagen-based biomaterial; collagen type I and II or collagen/glycosaminoglycan (GAG) composites and synthetic such as polyglycolic acid (PGA) and poly-lactic acid (PLLA), or their composite mixture, PLGA (poly D,L-lactic-co-glycolic acid), have been introduced as potential cell-carrier substances for cartilage repair. They have shown that cartilage-specific extracellular matrix (ECM) components such as type II collagen and GAG play a critical role in regulating expression of the chondrocytic phenotype and in supporting chondrogenesis both in vitro and in vivo.
Chitosan is the alkaline de-acetylated product of chitin and a family of poly-D-glucosamine units that vary in their degree of deacetylation and molecular weight. Many investigators suggested that chitosan might be considered as a structural biomaterial for the repair of connective tissues because of its structural similarity to GAGs found in the extracellular matrix. Chitosan and some of its degraded products can be involved in the synthesis of the articular liquid components such as chondroitine, chondroitine-sulfate, dermatane-sulfate, keratane-sulfate and hyaluronic acid (HA). These substances are necessary for nutrition of the cartilage. The fact that chitosan is polycationic, and its structure is similar to hyaluronic acid which is an important molecule of ECM of articular cartilage has a particular importance for cartilage tissue engineering. Use of chitosan-based matrices in the tissue engineering of hyaline cartilage has been recently reviewed. Lahiji et al. (2000) showed that chondrocytes which are cultured on chitosan films maintain differentiated phenotype and express cartilage specific ECM proteins such as type II collagen and sulfated proteoglycan. Studies exploring the use of chitosan to potentiate neochondrogenesis have shown the ability of chitosan to promote the maintenance of the chondrocyte phenotype and biosynthesis of cartilage specific ECM components when grown on chitosan films (Lahiji et al, 2000). Chitosan has also been shown to potentiate the differentiation of osteoprogenitor cells and may have also enhanced bone formation.
Hyaluronic acid plays a vital role in many biological processes such as tissue hydration, proteoglycan (PG) organization in the ECM, and cell differentiation. It is also a component of healthy articular cartilage. Patti et al. (2001) showed that HA improved in vitro substrate adhesion ability and proliferative activity of human cartilage cells. HA also improved clinical function in early arthritis (Patti et al., 2001).
Further, fibroblast growth factor (FGF) is a strong mitogen for connective tissue cells and MSCs (J. Cell Biol. 100, 477-485, 1985). During cell expansion, FGF inhibits the formation of thick F-actin structure to maintain chondrogenic potential of articular chondrocytes (Exp. Cell Res. 253, 681-688, 1999). Also, FGF is known to maintain multifamily differentiation of MSC throughout numerous mitoses.